1. Technical Field
This disclosure relates to a semiconductor device, and more particularly to a semiconductor device with a driver circuit capable of supplying electricity to a plurality of light emitting diodes.
2. Discussion of the Background
Recent advances in semiconductor technology have led to development and application of enhanced light emitting diodes (LEDs). Particularly, developments of LEDs with increased brightness and blue LEDs have expanded the use of LED technology.
LEDs with high brightness are used in various illumination devices, for example, liquid crystal display (LCD) backlighting and indicator lamps for automobiles. The development of blue LEDs has made possible a full color display using red-green-blue (RGB) LEDs.
Typically, an LED device for illumination or display contains a plurality of LEDs. For example, an LCD panel uses a plurality of white or multi-color LEDs for backlighting. Such an LED device includes an LED driver circuit that serves to control an electric current supplied to drive the plurality of LEDs (hereinafter referred to as drive currents).
FIG. 1 is a layout diagram illustrating a background LED driver circuit 200. The circuit 200 includes a first transistor array A1, a second transistor array A2, wires 201, 202, 203, 204, 205, and 206, connection pads 221, 222, 223, 224, 225, and 226, a pair of source pads 231 and 232, and thick wires 233, 234, and 235.
The first transistor array A1 is disposed substantially along one side of the circuit 200, including a first transistor 211, a second transistor 212, a third transistor 213, and a fourth transistor 214. The second transistor array A2 is disposed substantially along another side of the circuit 200, including a fifth transistor 215 and a sixth transistor 216. The transistors 211 through 216 may be N-channel metal oxide semiconductor (NMOS) transistors, for example, for driving a plurality of LEDs (not shown).
The plurality of LEDs are respectively connected to the corresponding drains of the transistors 211 through 216 via the connection pads 221 through 226.
The pair of source pads 231 and 232 are located between the forth transistor 214 and the fifth transistor 215 and coupled via the thick wire 233.
The wires 201 through 204 respectively connect sources of the first through fourth transistors 211 through 214 to the thick wire 234 extending along the first transistor array A1. The wires 205 and 206 respectively connect sources of the fifth and sixth transistors 215 and 216 to the thick wire 235 extending along the second transistor array A2.
The thick wire 234 is connected with the source pad 231, and the thick wire 235 is connected with the source pad 232.
An electric current for each of the plurality of LEDs is supplied from one of the pair of source pads 231 and 232. The electric current passes through one of the thick wires 234 and 235 to flow in one of the transistors 211 through 216 via corresponding one of the wires 201 through 206. The electric current is then supplied to corresponding one of the plurality of LEDs via corresponding one of the connection pads 201 through 206.
Referring to FIG. 2, an exemplary circuit diagram of the background LED driver circuit 200 of FIG. 1 is described. In FIG. 2, the circuit 200 includes LEDs D201 through D206, the first through sixth transistors 211 through 216, the connection pads 221 through 226, first resistors R11a through R16a, second resistors R21a through R26a, the pair of source pads 231 and 232, a power supply Vdd, and a bias terminal Vb.
The power supply Vdd is connected to anodes of the LEDs D201 through D206, and the connection pads 221 through 226 are respectively connected to cathodes of the LEDs D201 through D206.
The bias terminal Vb is connected to gates of the transistors 211 through 216, which are biased at a bias voltage Vb. The power supply Vdd provides each of the LEDs D201 through D206 with a drain current corresponding to the bias voltage Vb.
The first resistors R11a through R16a and the second resistors R21a through R26a both represent wire resistance. The wire resistance is an electrical resistance of a wire material (e.g., a metal material) used to form the circuitry.
Namely, in FIGS. 1 and 2, the first resistors R11a through R16a represent wire resistance associated with the wires 211 through 216. The second resistors R21a through R26a represent wire resistance associated with the thick wire 234.
Even though the first and second resistors R11a through R16a and R21a through R26a have relatively low resistance in general, the wire resistance causes voltage drop when an electric current of, for example, several hundred milliamperes passes through wire.
The voltage drop across each of the first and second resistors R11a through R16a and R21a through R26a affects gate-source voltage of the transistors 211 through 216, which is closely related to drain current of each transistor.
In the circuit 200, the drain current of each of the transistors 211 through 216 is the drive current supplied to drive each of the LEDs D201 through D206. Therefore, the wire resistance as represented by the first and second resistors R11a through R16a and R21a through R26a is related to the brightness of the LEDs D201 through D206.
In the circuit 200, the wire resistance represented by each of the resistors R11a through R16a varies depending on length and width of each wire. The wires 201 through 206 have an extremely short, substantially common length and width, such that the first resistors R11a through R16a have a substantially same low resistance to each other. Since each of the wires 201 through 206 carries an amount of electric current supplied to corresponding one of the LEDs D201 through D206, the voltage drop across each wire is substantially identical to each other.
On the other hand, the thick wires 234 and 235 have relatively high resistance due to wire length. The resistance represented by the second resistors R21a through R26a is several or several dozen times more than the resistance represented by the first resistors R11a through R16a. 
The thick wire 234 carries electric currents supplied to the LEDs D201 through D204 and the thick wire 235 carries electric currents supplied to the LEDs D205 and D206. Even though the resistance of the thick wires 234 and 235 represented by the resistors R21a through R26a is substantially uniform, the voltage drop varies according to the distance from the source pad, i.e., the resistor nearer to the source pad causes a higher voltage drop.
In addition, the number of resistors through which the electric current for one of the LEDs D201 through D206 passes varies depending on the position of the transistor in relation to the corresponding source pad.
In FIG. 2, the electric current supplied to one of the LEDs D201 through D204 passes through corresponding one of the first resistors R11a through R14a and at least one of the second resistors R21a through R24a to flow in the source pad 231. Similarly, the electric current supplied to one of the LEDs D205 and D206 passes through corresponding one of the first resistors R15a and R16a and at least one of the second resistors R25a and R26a to flow in the source pad 232.
For example, the electric current supplied to drive the LED D201 passes through five resistors, i.e., the first resistor R11a and the second resistors R21a through R24a, to flow in the source pad 231. The electric current supplied to drive the LED D204 passes through two resistors, i.e., the first resistor R14a and the second resistor R24a, to flow in the source pad 231.
Therefore, two factors cause fluctuations in the brightness of the LEDs D201 through D206 in the driver circuit 200. The variation in number of resistors through which the drive current passes, together with the variation in voltage drop provided by each resistor, translates into the variation in drive current, which results in the differences in the brightness of the LEDs D201 through D206.
The differences in the brightness of the plurality of LEDs or non-uniformity in LEDs intensity may affect performance of the LED device, degrading display quality and/or color reproducibility. The non-uniformity in LEDs intensity may be reduced by accurately providing drive currents of equal intensity to the plurality of LEDs.
An approach to reduce the variation in drive current is to directly connect each transistor to a corresponding source pad using a separate wire. Such an approach may simplify the driver circuit by removing resistors through which electric currents for different destinations commonly flow, that is, the thick wires 234 and 235 of FIG. 1.
FIG. 3 is a layout diagram illustrating another background LED driver circuit 300. The driver circuit 300 includes a first transistor array B1, a second transistor array B2, wires 301, 302, 303, 304, 305, and 306, connection pads 321, 322, 323, 324, 325, and 326, a pair of source pads 331 and 332, and a thick wire 333.
The first transistor array B1 includes a first transistor 311, a second transistor 312, a third transistor 313, and a fourth transistor 314. The second transistor array B2 includes a fifth transistor 315 and a sixth transistor 316. The transistors 311 through 316 may be NMOS transistors, serving as drives for LEDs (not shown).
In the circuit 300, components including the transistors 311 through 316, the connection pads 321 through 326, the pair of source pads 331 and 332, and the thick wire 333 are located in a similar manner as in the circuit 200.
The wires 301 through 304 respectively connect sources of the first through fourth transistors 311 through 314 to the source pad 331. The wires 305 and 306 respectively connect sources of the fifth and sixth transistors 315 and 316 to the source pad 332.
The wires 301 through 306 are of substantially uniform width. Each wire has a particular length corresponding to the distance between the corresponding transistor and the source pad connected thereto. Consequently, there exists a variation in wire resistance due to the varying lengths between the wires 301 through 306, resulting in the variation in drive current for the plurality of LEDs.
To reduce variation in performance among a plurality of electric components in a semiconductor device, various background techniques have been proposed.
In a semiconductor integrated circuit (IC) device that employs one of these techniques, a signal source supplies clock signals to a plurality of circuits with a common wire whose width decreases with relative distance from the signal source. As the resistance increases with the decreasing width of the common wire, the variation in voltage may be reduced to a certain degree while slight differences of voltage are not completely removed.
In a pattern layout method for an LCD panel that employs another technique, terminals are connected by through-holes and wires with common resistance. Such a pattern layout method is configured to regulate time delay within a driver circuit, in which the variation in brightness of multiple LEDs still remains unsolved.